In the copending application of Sanders and Forsyth, Ser. No. 544,397, filed Oct. 21, 1983, there is described a dot matrix print head designed for high speed operation having rugged construction and low cost. In the assembly of such dot matrix print heads it is essential, to achieve the design objectives of high performance and low cost, that the armature carried by the pin driving spring be accurately positioned with respect to the armature receiving hole in the solenoid. There are certain manufacturing tolerances in devices of this type where only a few thousandths of an inch can make a tremendous difference in the reliability and smooth operation of the print pin. This can require permanent jigs and fixtures which are expensive and sometimes not completely satisfactory.
In dot matrix print heads of the type described in the above mentioned patent application, the print head is moving constantly across the sheet and the firing of each individual print wire is controlled by a computer in accordance with the predicted position of the particular print wire across the sheet at any given instant of time to provide a small portion of the desired character. Since the print head is capable of operating at 3,000 impulses per second for each print wire, and since the print head may be moving across the sheet at 52 inches per second, each print wire must make its impact with the sheet within a time frame of only 40 microseconds if it is to form the desired character. Any impact outside of the 40 microseconds window will distort the printed image.
As a result of this critical time dependency of the impact with respect to motion of the print head, it is extremely critical that each print wire have the same response time to the firing pulse. This means that, insofar as is mechanically possible, each wire driving armature must be precisely centered with respect to its solenoid and the gap should be as small as possible consistent with reasonable manufacturing techniques. If the armature is not precisely centered, it may rub against the side of the hole, thereby enormously increasing frictional force to be overcome in moving the armature. Also it will otherwise change the response time. When each print wire is designed of the same mass, each armature has the same mass and each is assembled in identical solenoid, each one can have a response time within 20 microseconds of each other print wire so that optimum printing quality will be obtained with electrical firing pulses of the same length sent to each printing solenoid in proper sequence.
Accordingly it is the object of the present invention to provide a simple and inexpensive method for assembling print driving springs carrying an operating armature in fixed coaxial relation to its driving solenoid.